The production of nickel, cobalt and other metal powders by pressurized hydrogen reduction of aqueous ammoniacal sulfate solutions is a commercial practice pioneered by Sherritt Gordon Mines in Canada. In the process, it has been found that reduction kinetics are improved by the presence of particulate seed material such as fine nickel or cobalt powder. Such powders may come from many sources and may be introduced into the autoclave to act as nuclei for powder formation. U.S. Pat. No. 2,796,343 mentions the production of fine metal particles for use as seed by operations such as grinding of larger particles; precipitation by such reducing agents as hydrophosphite, hydrazine and the like which are stronger than hydrogen, decomposition of nickel carbonyl and the limited reduction of acidic solutions. U.S. Pat. Nos. 2,734,821, 2,796,342 and 2,796,343 describe "self-nucleating" solutions in which agents such as stannous, cerous, manganous, ferrous, titanous, vanadous and chromous salts are added.
U.S. Pat. No. 4,545,814 shows that strong reductants such as metal hydrides, metal borohydrides and metal borides promote reduction kinetics during the pressurized hydrogen reduction.
Known cobalt and nickel powders produced by the hydrogen reduction method are usually coarse, spherical powders since the usual practice is to repeatedly precipitate further metal upon the initially formed particles by further "densifications".
There are needs in the art for ultrafine powders, particularly of cobalt, for purposes such as cementing carbide tools. Such powders may have a grain size of 2 to 3 micro-meters (.mu.m).
Present production methods used for such powders are shrouded in mystery, but are believed to entail solution of cobalt metal in hydrochloric acid, precipitation of the dissolved cobalt as cobalt oxalate or carbonate, filtration, washing, drying, decomposition to cobalt oxide and reduction to cobalt metal by hydrogen at elevated temperature. The resulting powder is then milled and screened and fractions of graded particle size are marketed. The method is complex and capital intensive.
Production of cobalt powder for use in cemented carbides requires much more than control of particle size. Particle morphology apparently is also important. Thus, in the production of hard metals, such as cobalt-cemented tungsten carbide, tungsten carbide powder of graded size between approximately 1 .mu.m and 10 .mu.m in particle size is blended with ultrafine cobalt powder, generally by ball milling with the addition of alcohol. The blended powder is mixed with a lubricant such as paraffin, dried and pressed to shape, dewaxed and presintered and then sintered at high temperature in a controlled protective atmosphere. The purpose of grinding tungsten carbide powder together with cobalt powder is to cover the surface of carbide particles evenly with cobalt. It is particularly important that the cobalt powder should be sufficiently uniform and finely dispersed to obviate the formation of cobalt agglomerates which may cause defects in the structure of the hard metal during sintering. Since the tungsten carbide powder is very fine grained (1 to 10 .mu.m), the cobalt powder should be at least as fine grained as the carbide powder.
During hydrogen reduction of cobalt from a sulfate solution, sulfuric acid is generated. The amount of base added, such as ammonium hydroxide, sodium hydroxide or potassium hydroxide, should be sufficient to neutralize the acid generated, otherwise the reduction will not be complete. EQU CoSO.sub.4 +H.sub.2 .fwdarw.Co.degree.+H.sub.2 SO.sub.4 EQU H.sub.2 SO.sub.4 +2NH.sub.4 OH.fwdarw.(NH.sub.4).sub.2 SO.sub.4 +2H.sub.2 O
In U.S. Pat. No. 4,545,814, the Examples show that all ammonia required for acid neutralization was added to the sulfate solution initially, thereby producing a strongly ammoniacal solution, although the statement is made that ammonia can be added during the reduction stage.
The present invention resulted from close study of the reactions described in U.S. Pat. No. 4,545,814 and is directed to an improved process in which the reduction time is greatly decreased with minimal consumption of reagents and minimal sulfur contamination of the fine metal powder product.